Dark conductivity in the x and alpha polymorphs of lithium phthalocyanine

This study focuses on the correlation existing between the structure and the continuous dark conductivity in single crystals, compacted powders and thin films of the lithium phthalocyanine radical (PcLi). The dc conductivity at room temperature measured in PcLi compacted powders is of about 2-4 . 10(-5) (Ohm cm)(-1) for the x structure and 1-2 . 10(-4) (Ohm cm)(-1) for the alpha structure. The temperature dependence of the conductivity measured in the range 130-300 K hints at an activated-like behaviour with activation energies in the range 0.1-0.2 eV depending on the structure. PcLi thin films grow either in the x or the alpha structure depending on the substrate temperature (T-s) during deposition on the glass substrate. The conductivity shows a clear and strong dependence on the substrate temperature T-s and increases between 1 . 10(-6) (Ohm cm)(-1) for T-s = 25 degrees C and 5 . 10(-4) (Ohm cm)(-1) for T-s = 200 degrees C. Annealing of the PcLi thin films obtained for T-s = 25 degrees C at 200 degrees C under vacuum results in a clear increase of the conductivity by more than one order of magnitude. The conductivity is independent on the presence of oxygen in powdered samples and single crystals, indicating the intrinsic nature of the semiconducting behaviour. Contrary to alpha-PcLi thin films (T-s similar to 200 degrees C), the conductivity in PcLi x-form thin films (25 degrees C < T-s < 125 degrees C) is sensitive to the presence of ambient atmosphere (sigma(air)/sigma(vac) similar to 2.3 at 300 K for T-s = 24 degrees C). This weak variation of the conductivity cannot be attributed to extrinsic charge carrier generation as observed for other metallophthalocyanines. (C) 1998 Elsevier Science S.A. All rights reserved.